The capacity of a wireless communication system depends on the capacity that the down-link links can support to a large extent. The traditional single-antenna transmission technology has been far unable to adapt to worse and worse wireless transmission environment. Advanced down-link transmission technology has played an important role in improving system performance and increasing capacity. Thus, various transmission technologies (related to time, multipath, space, and the like), which are able to obtain diversity gain, have been paid closer and closer attention and applied more and more in the recent years.
Two-antenna transmission diversity technology used for a base station (referred to as “Node B”) in a code division multiple access (CDMA) system is determined by 3GPP Rel'99 protocol standard. It mainly uses the characteristics that signal fading during signal transmission is not associated with the fact that different antennas are used if the antennas are spaced apart with a distance larger than 10 wavelength of a carrier wave. So, the signals transmitted by the respective antennas are typically combined by employing a maximum ratio, thereby preventing multipath signal fading and enhancing system performance.
One of the wide-band features of a wide-band code division multiple access (WCDMA) system is to use multipath diversity of wireless transmission channels. The transmission antenna with increased number of Node B can be equivalent to increasing the number of propagation multipaths, which enables to obtain a larger gain of multipath diversity. Different technologies of multiple-antenna transmission diversity have been proposed in recent years.
Space diversity gain can be provided by an antenna array. Based on the same statistic characteristics of a domain of up- and down-link channels, the capacity of a wireless communication system can also be increased by formation technology which forms a wave beam of a down-link antenna array, wherein an evaluated up-link destination object angle (DOA) is utilized. However, the diversity gain obtained therein is limited.
Multiple-antenna closed-loop diversity array requires lager spaces between antenna units, normally larger than ten times of the wavelength. The multiple-antenna closed-loop diversity array will occupy more space. Multiple-antenna closed-loop diversity array requires higher feedback bits. If the original feedback rate does not change, the channel fading becomes worsen with higher speed, and the performance will be deteriorated. Furthermore, although multiple-antenna closed-loop diversity array has an effect for resisting the multipath fast fading, it cannot provide antenna gain and suppress the multiple access interference signals effectively.
It is often required by the multiple-antenna closed-loop diversity array that a user equipment (UE) calculate not only the short period fast variation weight vectors, but also the long period slow variation beam weight vector sets in each time slot. In one aspect, the complexity of UE is increased excessively while more feedback bits are required for feeding back the long period slow variation beam weight vector sets, even though the low feedback rate may be used by the slow variation beam weight vector sets.
The technology for forming down-link beams of an antenna array is still not perfect. A representative down-link method is based on that the statistic characteristics of the up- and down-link channels are the same, and DOA evaluated in an up-link is used to transmit in a down-link. In a frequency division duplex (FDD) mode, the difference between operation frequencies of up- and down-links would require a correction or adaptation system. Moreover, although the above method has space diversity gain, it does not resolve the problem of multipath fast fading.